This invention relates to turbomachines which include self-aligning vanes.
The efficiency and stability of a compressor is dependent upon the means for converting the kinetic energy of the air leaving the impeller into static pressure. Most high-performance centrifugal stages used a fixed-vane diffusion section to accomplish this kinetic energy conversion. The low flow limit for the compressor corresponds to the onset of a surge or stall condition which occurs as the fluid flow from the impeller becomes more tangential as the fluid flow decreases. This produces a large flow angle and magnitude with respect to the leading edge of the fixed diffuser vanes, creating a violent instability in the stage. The high flow or max-flow limit corresponds to a choke condition caused as increasing fluid flow from the impeller becomes more radial and finally chokes the diffuser throat with very large kinetic energy loss. The design point is generally established such that the fluid flow meets the diffuser vane leading edge with zero or small incident angle.
Various techniques are used to increase the range between the surge and choke limits, especially when the compressor is utilized in a high performance gas turbine engine or turbocharger. One technique to increase the compressor's range is to utilize power actuated movable vanes which are driven by an external motive system, either between closed and open positions, or infinitely, as disclosed for example in ASME paper 68-GT-63, entitled "Variable Geometry Gas Turbine Radial Compressors", by C. Rogers. Another example is disclosed in U.S. Pat. No. 3,588,270 to Albin Boelcs in which a centrifugal compressor has a diffuser with two coaxially arranged rows of rotatable guide vanes in which the pivot axis of the vanes are displacable in relation to one another by externally driven annular vane disks. The driven movable vanes, and other apparatus for defeating a surge condition, have been controlled by various control systems having an input transducer for sensing the onset of a surge condition. In U.S. Pat. No. 2,566,550 to R. Birmann, a freely rotatable flag or vane is located upstream of the entrance edges of one pair of fixed diffuser vanes to indicate the onset of a surge condition as the flag moves to a more tangential position, actuating a motive mechanism for preventing the surge condition.
All such prior techniques are complex and require the accommodation of power actuating motive means and surge sensors as well as a control system. Furthermore, such techniques are not readily adaptable to solve the problem of changing fluid direction in other stages of a gas turbine engine such as the second stage nozzle of a two-stage turbine.